The need to reduce nitrogen- and carbon-based emissions while maintaining economic operation is a concern wherever carbonaceous fuels are burned. The requirements of each system for efficiency of fuel consumption must, of course, be considered in order to have a practical system.
In U.S. Pat. No. 3,599,427 to Jones et al, there is described a two stage catalytic system for treating the exhaust gases of mobile internal combustion engines. In the first catalytic stage, hot exhaust gases directly from an engine are treated at a high temperature to oxidize carbon monoxide and unburned hydrocarbons. The resulting exhaust gases are then cooled and passed through a separate, second catalytic stage to reduce levels of nitrogen monoxide. Prior to contact with the second stage catalyst, ammonia gas and other compounds such as urea, ammonium hydroxide, ammonium carbonate, and hexamethylenetetramine, are mixed with the exhaust gases. Upon contact with the second stage catalyst, nitrogen oxides are reduced to produce nitrogen and water.
In U.S. Pat. No. 3,846,981, Pacztowski discloses a more detailed, controlled two-stage catalytic system. The operating temperatures for the second catalytic stage where ammonia is utilized, are preferably within the range of from 275.degree. F. to 900.degree. F. This process and that of Jones et al unfortunately depend on the use of catalysts which create additional costs in terms of initial investment and servicing requirements.
In U.S. Pat. No. 3,900,554, Lyon discloses a non-catalytic system for reducing nitrogen monoxide (NO) in a combustion effluent. Lyon discloses that ammonia and specified ammonia precursors, including ammonium carbonate also disclosed by Jones, et al, or their aqueous solutions, can be injected into the effluent for mixing with the nitrogen monoxide at a temperature within the range of 1600.degree. F. to 2000.degree. F. In one embodiment of the disclosed process, a reducing agent, such as hydrogen gas or various hydrocarbons, can be mixed with the effluent to permit the reduction reaction to occur at temperatures as low as 1300.degree. F., thereby assuring avoidance of high temperature oxidation of ammonia to nitrogen monoxide. Lyon points out that at temperatures above 2000.degree. F., the use of ammonia was counterproductive--increasing NO rather than decreasing it.
Unfortunately, large industrial boilers operate at temperatures significantly above 2000.degree. F., and access to the interior of the heat exchangers where the 1600.degree. F. to 2000.degree. F. temperature exists following the flame zone of the boilers is not practical without major redesign due to exterior water jacketing and interior water tubes. At the exhaust end of the boilers, the temperature is reduced far below the minimum temperature of 1300.degree. F. which can be used when a reducing agent is employed. Thus, the effective temperature range cannot be readily accessed for non-catalytic operability of Lyon's teaching in many large industrial boilers and certain other NO.sub.x -producing combustion equipment.
In U.S. Pat. No. 3,961,018, Williamson discloses the purification of acid gas-containing streams at low temperatures approaching ambient. Williamson discloses contacting the gas stream with an amine vapor in sufficient concentration such that its partial pressure is at least 5% of the total pressure of the gas stream. This system thus requires large amounts of the treating gas and requires equipment for separating that gas from the effluent upon completing the treatment.
In a somewhat different environment, Goldstein et al, in U.S. Pat. No. 4,061,597 indicate that temperatures within the range of 1000.degree. to 1300.degree. F. are effective when using urea for reducing brown fumes caused by nitrogen dioxide (NO.sub.2) from catalyst treatment effluents. One example in the patent employs a 30 weight percent aqueous solution of urea. Again, however, the temperature range of 1000.degree. F. to 1300.degree. F. is not practical for treatment of effluents from many types of combustion equipment.
In U.S. Pat. No. 4,325,924, Arand et al disclose the non-catalytic urea reduction of nitrogen oxides in fuel-rich combustion effluents. They indicate that under fuel-rich conditions, aqueous solutions of urea at concentrations of greater than 10%, and preferably greater than 20%, are effective nitrogen oxide reducers at temperatures in excess of 1900.degree. F. Unfortunately, this effluent from staged combustion results in the production of high levels of carbonaceous pollutants.
In U.S. Pat. No. 4,208,386, on the other hand, Arand et al disclose that for oxygen-rich effluents, the temperature is in the range of from 1300.degree. F. to 2000.degree. F. for urea added dry or as a solution in water alone or with an alkanoic solvent. The use of the alkanoic solvent is said to enable reduction of the effective operating temperature to below 1600.degree. F. No function, other than carrier for the urea, was disclosed for the water.
Operation under fuel-rich conditions has the disadvantages that combustion has been incomplete and carbon-based pollutants are excessive. Thus, despite the apparent ability of Arand et al to add the urea solution to fuel-rich effluents at temperatures above 1900.degree. F. for reduction of nitrogen-based pollutants, this fuel-rich operation has economic and environmental penalties. And, operation under oxygen-rich conditions to achieve the desirable economies of fuel utilization and reduced carbon-based pollutants, causes practical difficulties in supplying the urea, ammonia or other useful material to a boiler under conditions where it will have its intended effect of reducing the levels of nitrogen oxide pollutants. Moreover, the present invention shows that urea is not as effective as would be desired in reducing levels of nitrogen oxides.
Accordingly, there is a present need for a process which enables the more efficient reduction of nitrogen-based pollutants while operating under efficient oxygen-rich conditions which minimize carbon-based pollutants.